In mobile radio systems of the second and third generation, such as the Global System for Mobile Communications (GSM) and the Universal Mobile Telecommunications System (UMTS), Non-Transparent (NT) data bearers are provided that offer an error-free data transfer service to the user. The data transfer service is based on the Radio Link Protocol (RLP) and the Layer-2 Relay (L2R) Character Oriented Protocol (COP). The RLP function offers an Automatic Repeat Request (ARQ) protocol that extends from the mobile station (MS) to the network Interworking Functions (IWF) in the Mobile-services Switching Centre (MSC) in order to detect errors by means of a Forward Error Correction (FEC) procedure and RLP's Frame Check Sequence (FCS) for each transmitted RLP frame, where an RLP frame represents an RLP Protocol Data Unit (PDU), and to eliminate errors by repeating the transmission of the frame under exploitation of the time-variance of the transmission medium. The L2R function converts the layer-2 protocol of the MS into a COP that uses transmission protected by an RLP.
The RLP is controlled by several parameters such as acknowledgement, reply and re-sequencing timers or the number of retransmission attempts or required window sizes, that either are assigned default values or can be modified by the user or network e.g. by means of AT commands. If a change of parameters is initiated in either the MS RLP entity or the MSC RLP entity, the desired parameters are signalled to the corresponding peer RLP entity via exchange IDentification (XID) frames, which are RLP frames (PDUs) in which the information field is interpreted as exchange identification instead of data. To start negotiation, an XID command frame will be signalled. The peer entity confirms the value of each parameter by returning the value within an XID response or offering lower or higher values of the parameter in its place depending on the sense of negotiation of the parameter.
The RLP may use one physical link (single-link) or from 1 up to 4 sub-streams on one or more physical links (multi-link). The multi-link version of the RLP protocol is only applicable in GSM and not in UMTS.
Among said control parameters of the RLP protocol, the acknowledgement timer T1 associated with the transmitting RLP entity indicates the re-transmission period after which the re-transmission of a not-acknowledged frame may be started. Due to ARQ in combination with FEC, each received RLP frame is checked for correct/incorrect reception at the receiving RLP entity, and correct/incorrect reception is signalled back to the transmitting peer RLP entity. The timer T1 defines the maximum time period starting with the transmission of an RLP frame within which a correct/incorrect acknowledgement of the transmitted RLP frame is expected. An expiration of the timer T1 causes the retransmission of the frame because the acknowledgement of the sent frame was not received in time.
Among said control parameters of the RLP protocol, the timer T2 associated with the receiver indicates the maximum permissible period the receiving RLP peer entity is allowed between the reception of a frame and the transmission of the acknowledgement message.
Among said control parameters of the RLP protocol, the re-sequencing timer T4 guards the maximum difference between the delays of frames transmitted on different physical links within the multi-link RLP protocol. The timer T4 defines how great the variation of the transmission delay of all physical links can be. If received frames are out of sequence, the receiver waits for the duration of timer T4 for the missing frames before starting any recovery actions. Concluding, in multi-link operation (e.g. GSM), T1>T2+T4+2*TD has to hold, were TD is the transmission delay between MS and MSC, whereas in single-link transmission (e.g. UMTS), T1>T2+2*TD has to hold.
In both GSM and UMTS, the transmission of the PDUs/frames of the RLP is performed by lower layers of the protocol stack. The delay characteristics of the RLP frames are thus at least dependent on the delay characteristics of the physical layer, i.e. the lowest layer in the protocol stack. When considering the transmission of RLP PDUs from an MS RLP entity to an MSC RLP entity in GSM, in the physical layer the transmission paths between the MS and the Base Transceiver Station (BTS), between the BTS and the Base Station Controller (BSC) and between the BSC and the MSC have to be taken into account. In UMTS, basically the same propagation paths are encountered, where the MS corresponds to the User Equipment (UE), the BTS corresponds to the Node B and the BSC corresponds to the Radio Network Controller (RNC). In the sequel, GSM notation will be used to identify the components of both mobile radio systems.
The BTS-BSC and BSC-MSC interfaces are usually realised by lower-delay connections such as Time Division Multiplex (TDM) connections (e.g. ISDN primary rate). However, also higher-delay connections such as Internet Protocol (IP), e.g. in a Distributed Radio Access Network (DRAN) environment or in an IP-based GSM Intranet Office (GIO) environment, or satellite connections may be applicable. It is easily understood that depending on the delay characteristics of the BTS-BSC or BSC-MSC connection, especially the timers T1 and T4 of the RLP have to be adapted accordingly to assure proper operation of the mobile radio system.
The applicant's international patent application WO 02/25888 A2 discloses one approach for an adaptation of RLP timers. WO 02/25888 A2 sets out from the fact that, in a typical GSM physical link, the transmission delay is within a tightly bounded range so that the RLP entities will use default values for the RLP timers based on the expected characteristics of the physical link. For the case when unexpectedly large delays occur, e.g. in an IP-based GSM office environment, an XID proxy unit is proposed as an additional negotiation unit that monitors and verifies XID commands sent between the MS and MSC entities. The XID proxy has knowledge of the maximum delay values for the physical link between the MS and the MSC. Based on this information, it has the capacity to intervene in the process of negotiation of T1 timer values between the MS and the MSC with the aim of ensuring that the value that is settled upon is large enough to cope with transmission delays that might be beyond the expectations (the default or offered timer values) of the MS and MSC.
In this prior art approach, the XID proxy is only activated when it “sniffs” the passing of an XID negotiation message between the RLP entities of the MS and the MSC. For this to happen, it is required that a new NT data call is set up within the higher-delay network, so that standard RLP timer negotiation between the RLP entities of MS and MSC is initiated via XID frames.
However, the prior art approach fails to adapt the RLP parameters to the delay characteristics of a higher-delay network if the NT data call was set up—and thus parameterised with a smaller timer during the initial RLP timer negotiation—in a lower-delay network and is subsequently handed over to the higher delay network, where no re-negotiation of the RLP timers takes place and thus the XID proxy is not activated. Such a situation occurs if a MS is associated with a first BTS that is connected to its MSC via TDM, and then is handed over to a BTS that is connected to its MSC via IP (where both MSCs can well be the same).
A similar problem arises when a hand-over of a UMTS-based NT data call to a GSM-based call occurs and RLP parameters that are essential in the GSM system, but not essential in the UMTS system, e.g. the RLP timer T4, are not negotiated or re-negotiated upon entry in the GSM system, so that default values are used in the GSM system instead of using values that have been adapted or selected for particular use in the GSM system.